Method of making an electrical unit



Dec. 7, 1965 w. R. BELKQ, JR. ETAL 3,222,173

METHOD OF MAKING AN ELECTRICAL mm:

Filed May 15, 1961 3 Sheets-Sheet 1 C/EQ/ IN VEN TORS 14 a001, 1?. 3624 0, Jr B1$q r' C, Vanda/"mark 1965 w. R. BELKO, JR., ETAL 3, ,173

METHOD OF MAKING AN ELECTRICAL UNIT Filed May 15, 1961 3 Sheets-Sheet 2 Z0 28 UN l|| 0 COATED WlTH AFTER EXPOSURE AFTER KEHOVAL AFTER LIGHT SENSITIVE AND TO LIGHT ETCHING RES \ST MATERIAL. UNEXPOSED COATING Fly? 7 4 77 2 4M I1 I. I

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MATER! A L- OF UN EXPOSED HATER IAL.

INV EN TORS William E. fieiko, Jr: 1% 5 C Vanda/mark ATTORNEYS 7, 1965 w. R. BELKO, JR. ETAL 3,

METHOD OF MAKING AN ELECTRICAL UNIT Filed May 15, 1961 3 Sheets-Sheet 5 IN V EN TORS Wilda/11 E. 562160, Jr: Pqper C l andermar United States Patent 3,222,173 METHOD OF MAKING AN ELECTRICAL UNIT William R. Belko, Jr., Huntington, and Roger C. Vandermark, Sandy Hook, Conn., assignors to Vitramon, Incorporated, Monroe, Conn., a corporation of Delaware Filed May 15, 1961, Ser. No. 110,202 6 Claims. (CI. 96-35) This invention relates to the manufacture of electrical units, such as capacitors, resistors, toroidal inductors, solid state modules and the like. More specifically, it pertains to a method of providing patterns, having specific electrical characteristics, on opposite surfaces of a base member having other specific electrical characteristics.

In certain types of electrical units the electrical characteristics of the unit, at least in part, are dependent on the shape, size and relative location of patterns formed on opposite sides of a base member. For example, the capacitance of a capacitor is determined, not only by the dielectric constant of the dielectric material and the distance between the electrodes, but by the effective areas of the electrodes which may be defined as the superposed areas thereof. In certain capacitors, for instance, there is a flat base member of dielectric material which has the electrodes or plates formed in overlying relation on its opposite surfaces and, in such a unit, the capacitance is the result of the dielectric constant and thickness of the base member and the superposed area of the electrodes.

It will be readily understood that, in the manufacture of electrical units, such as capacitors or the like, in which the size, shape and relative location ofthe elements of the unit at opposite sides of a base member is critical, there is a necessity for positively controlling the formation of such elements.

Heretofore, many difierent methods have 'been resorted to for providing the elements or patterns on the opposite sides of a base member so that they are of a given predetermined size and shape and are located relative to one another in a predetermined manner. Such methods, while apparently successful, have inherent disadvantages in that they do not maintain absolute control over and correlation of all of the factors of size, shape and relative location of the elements. For instance, in some of the methods of manufacture, substantially absolute control is obtained over the size and shape of the patterns while control of the relative location of the patterns at opposite sides of the base member is not attained and in other methods proper indexing or positioning of the patterns relative to one another is obtained at the expense of a clearly defined pattern. Further, the methods known to the prior art often present problems when applied to the forming of patterns on fragile base members, such as the thin ceramic film used to form the dielectric base member in many capacitors and, additionally, the methods of the prior art are time consuming and expensive and require the services of highly skilled labor.

The present invention overcomes the problems aforenoted and has as an important object thereof to provide a method for making electrical units having sharply defined patterns or elements of predetermined size located in a predetermined manner relative to one another at opposite sides of a base member.

According to the invention, this object is achieved by coating the opposite surfaces of the base member with either a light or heat sensitive material, of the character which is adapted to harden on being exposed to the particular form of radiation to which it is sensitive, and then exposing the opposite surfaces to the radiation in such a manner as to harden the material to define predetermined patterns corresponding in size, shape and relative location to the elements desired to be formed on the base 3,222,173 Patented Dec. 7, 1965 member. Thereafter, the unhardened material is removed and the unit is treated by etching, spraying, firing or a combination of these procedures, as required by the particular physical make-up of the unit, so that on removal of the hardened radiation sensitive material patterns of predetermined size, shape and relative location remain on the opposite surfaces of the base member.

In one form of the invention a base member having specific electrical properties and provided on its opposite surfaces with a material having other specific electrical properties, is coated on its opposite surfaces with a radiation sensitive material and then exposed to radiation to harden the coatings in predetermined patterns. After removal of the unhardened coating the base member is treated with an etchant to remove the uncovered material of the other specific electrical properties, so that on subsequent removal of the hardened coating predetermined patterns of the material of the other specific electrical properties remain on the opposite surfaces of the base member in a predetermined relative location.

In another form of the invention, the coating of radiation sensitive material carries in suspension the material for forming the patterns on the base member. In this form of the invention, after the coating is exposed to radiation to harden it in areas corresponding in size, shape and relative location to the patterns it is desired to form, the unhardened material is removed, carrying with it the excess pattern forming material. Thereafter, the unit is fired to bond the remaining pattern forming material to the base member and burn off the hardened radiation sensitive material.

In still another form of the invention, the base member is coated with radiation sensitive material and then exposed to radiation so that the hardened areas are formed as borders around the unhardened coating which defines the desired patterns. In this form of the invention, on removal of the unhardened coating, the base member is sprayed, painted or similarly covered with material having different electrical properties than the base member, so that on firing the unit the sprayed on material bonds to the base member in the uncoated pattern areas and the excess is burned off with the hardened coating of radiation sensitive material.

It is another object of the invention to provide a method for making electrical units by exposing radiation sensitive coatings on the opposite sides of the base member to radiation through masking means which are predeterminately indexed to provide exposed areas of predetermined character.

Another, and more specific, object of the invention is to provide a method for making capacitor-s by simultaneously forming the electrodes in a predetermined size, shape and relative location on the opposite surfaces of a dielectric base member.

It is further an object of the invention to provide a a method of making electrical units which is readily adapted to high speed production requirements, at reduced cost, while maintaining absolute control over the final electrical characteristics of the unit.

Other objects and advantages will be apparent from the specification and claims when taken in connection with the attached sheets of drawings, illustrating one form of the invention, wherein like characters represent like parts, and in which:

FIGURE 1 is an exploded perspective view illustrating diagrammatically one method of making an electrical unit according to the invention;

FIG. 2 is a diagrammatic elevational view, partially in cross-section, illustrating the method of FIG. 1;

FIGS. 3 through 6 diagrammatically illustrate the step by step process of making an electrical unit according to one form of the invention;

FIGS. 7 through 10 illustrate diagrammatically the step by step method of making an electrical unit according to another embodiment of the invention;

FIGS. 11 through 14 illustrate diagrammatically the step by step method of making an electrical unit according to still another embodiment of the invention;

FIG. 15 is a plan view, partially broken away to illustrate internal construction, of an apparatus for use in practicing the method of the present invention; and

FIG. 16 is a side elevational view, partially in section, of the apparatus of FIG. 15.

Referring now to the drawings for a more detailed description of the invention, in FIGS. 1 through 6 one method of forming an electrical unit according to the invention is shown. While it will be understood that the concepts of the invention are applicable to the production of many types of electrical units, including capacitors, resistors, toroidal inductors and combinations thereof or the like, in the herein illustrated forms of the invention the method of making capacitors, of the type including a flat dielectric base member having electrodes on opposite surfaces thereof, is described.

As shown, a base member 20 of ceramic or other dielectric material is provided which has bonded to the opposite surfaces thereof electrically conductive layers 21 and 22 which may be silver or the like. The silvered surfaces 21 and 22 of the base member are coated, as at 23 and 24, with any well known radiation sensitive etchant resistant material of the type which is adapted to harden on exposure to the particular form of radiation to which it is sensitive.

As used herein, the term radiation will mean either light or heat and radiation sensitive material will be a material which is sensitive to at least one of these forms of radiation. The term light, as used herein, will include ultra-violet light as well as light in the visible spectrum. The term harden, when used herein to describe the condition of the radiation sensitive material after exposure to radiation, defines a condition which is resistant to the solvents that normally dissolve the radiation sensitive material prior to the exposure to radiation.

After the silvered surfaces of the base member 20 have been coated as above described with the radiation sensitive etchant resistant material, which for the purpose of the present illustration will be a light sensitive acid resistant material, the surfaces are masked to define patterns of a predetermined size and shape, which patterns are predeterminately located relative to one another on the opposite surfaces of the base member. While the concepts of the present invention are readily adapted to the manufacture of a single capacitor unit in which there is a dielectric base member having a single electrically conductive pattern or electrode on each of the opposed surfaces, in the illustrated form of the invention the production of a plurality of individual capacitor units is shown.

As illustrated in FIGS. 1 and 2, the means for masking the surfaces to define the predetermined patterns are photographic negatives and 26 which are adapted to be positioned closely adjacent or in engagement with the coated surfaces 23 and 24 respectively. Each of the negatives include a plurality of light transmissive portions 25a and 26a, each of which defines the pattern of one of the electrodes of an individual capacitor unit. It will be noted that the light transmissive portions 25a of negative 25 differ in shape from the light transmissive portions 26a of the negative 26 but when operably positioned adjacent their respective surfaces of the base member 20 are predeterminately located relative to one another. To put it another way, for example, the light transmissive portion 25b overlies in a predetermined manner, the corresponding light transmissive portion 26b, thereby resulting in the production of a capacitor, as will be hereinafter described, in which the electrodes on the opposite surfaces of the base member are predeterminately located relative to one another. The difference in the size and/ or shape of the light transmissive portions 25a and 26a, in the particular embodiment of the invention described, are for the purpose of facilitating the connection of leads to the individual capacitor units as is well known to the art. However, it is noted that in other embodiments of the invention the light transmissive patterns of opposed negatives may be identical.

It is necessary when positioning the negatives 25 and 26 adjacent their respective surfaces of the base member 20 to be absolutely certain that the light transmissive portions thereof are located relative to one another as required to achieve the desired results, namely the formation of patterns on opposite surfaces of the base member which are located in a predetermined relationship. Alignment of the light transmissive portions may be achieved by placing the negatives side by side and aligning the light transmissive portions visually and thereafter inserting the base member between the negatives or it may be achieved by mechanical means, as will be hereinafter described.

After the negatives or masking means have been positioned adjacent their respective sides of the base member 23 the assembly is exposed to a source of radiation for exposing those portions of the coatings 23 and 24 which are exposed to the source of radiation through the light transmissive portions of the negatives. In the illustrated form of the invention, an arc lamp 27 or the like provides the necessary light to expose the coatings defined by the light transmissive portions of the negatives for hardening the same. While in FIG. 2 the light source 27 is shown positioned adjacent only one side of the assembly and exposing only the coating 23 through the negative 25, it will be readily understood that both sides of the assembly may be exposed simultaneously. In fact, it is believed that, such simultaneous exposure is preferable since the number of steps in the operation is reduced, with an ultimate reduction in time required for the operation, and the length of time the exposed coatings are exposed is more closely coordinated.

Referring now to FIGS. 3 through 6, the steps of the method are illustrated diagrammatically to show more clearly the effect of exposure to radiation on the radiation sensitive coating and the subsequent steps in the method of making the finished capacitor. In FIG. 3 one side of the base member 20, having the electrically conductive material 21 thereon is shown coated with the light sensitive acid resistant material 23. In FIG. 4 the same view of the base member is shown after exposure of the coating 23 to light through the masking means as above described. It will be seen that the coating 23 has hardened as at 28 in the areas in which it was exposed, the hardened areas being in the form of patterns of predetermined size and shape and located relative to their corresponding pattern on the opposite surface of the base member in a predetermined manner. After the coatings have been exposed to light, and the portions 28 thereof have been hardened, the surfaces of the base member 20 are washed, asby dipping them in a bath or the like of a solvent of the radiation sensitive material to remove the unhardened coating 23, thus exposing the electrically conductive material 21 in those areas of the surface outside of the patterns 28, as shown in FIG. 5. Thereafter, as shown in FIG. 6, the surfaces of the base member are etched to remove the exposed electrically conductive layer and expose the bare surface of the base member 20. In the illustrated form of the invention, since the electrically conductive layer is silver, the etching material is preferably nitric acid. It will be understood, however, that if the electrically conductive layer were of some other material a different etching solution might have to be used. In the event that the required etching solution is alkali rather than acid it will, of course, be understood that the radiation sensitive material will be alkali resistant rather than acid resistant.

As final steps in the manufacture of capacitors, according to the form of the invention disclosed in connection with FIGS. 1 through 6, the hardened radiation sensitive material is removed, by burning it off or in any other manner known to the art, to expose the electrically conductive patterns 29 in FIG. 6 and then the base member is trimmed and cut into individual capacitor units by any means known to the art.

Referring now to FIGS. 7 through 10, another form of the invention is shown in which a bare dielectric base member 30 is coated with any well known radiation sensitive material, shown at 31. According to this form of the invention the radiation sensitive material 31, which may be a light sensitive material, has suspended therein particles of electrically conductive material, which may be silver or the like, and any flux known to the art for bonding the electrically conductive material to the base member. The electrically conductive particles which are shown at 32, are in sufiicient density to form the electrodes of a capacitor. After the surfaces of the base member 30 have been exposed to light, or heat in the case of a heat sensitive material, through a negative or other masking means, as described with respect to FIGS. 1 through 6, to form areas of hardened light sensitive coating in predetermined patterns, as shown at 33 in FIG. 8, the surfaces of the base member are washed with a solvent of the light sensitive material to remove the unhardened coating and the electrically conductive material carried therein and expose the bare surface of the base member 30, as shown in FIG. 9. It will be seen that there now remains on the surface of the base member, hardened patterns of light sensitive material which carry in suspension the particles 32 of electrically conductive material and flux. Thereafter, the base member is fired to melt the flux for bonding the electrically conductive particles to the surfaces of the base member for forming electrically conductive patterns 34 of predetermined size, shape and relative location. It should here be noted that during the firing operation the hardened light sensitive coating is burned off leaving the electrically conductive material behind. The base member is then trimmed and cut into separate capacitor units.

In FIGS. 11 through 14 still another form of the invention is shown. According to this form of the invention a bare dielectric base member 35 has the opposite surfaces thereof coated with any well known radiation sensitive material, shown at 36, which again for the purposes of illustration is a light sensitive material adapted to harden on being exposed to light. As with the preceding forms of the invention the opposite surfaces of the base member are masked by means of a photographic negative or the like, but, in the instant form of the invention the portions of the surfaces which are exposed to light define the borders around the areas on which the electrode patterns are to be formed, rather than the pattern areas themselves. In FIG. 12 a surface of the base member 35 is shown after being exposed to light through the masking means. Here it will be seen that all of the coated surface 36 is hardened with the exception of those areas 37 which were covered by the masking means and thus not exposed to the light. After the surfaces have been exposed to radiation they are washed by a solvent of the radiation sensitive material to remove the unhardened material from the pattern areas 37. Following this, the surfaces are sprayed, painted or similarly covered, as shown at 38 in FIG. 13, with an electrically conductive material, such as silver, containing a flux for bonding the electrically conductive material to the surface of the base member. While the surfaces may be completely covered with the electrically conductive material containing the flux, it is only necessary that the pattern areas 37 be completely covered. It should be here noted that while any flux which bonds the electrically conductive material to the base member may be used, it is preferred that the flux be of such a character that it will react with the hardened radiation sensitive material, in those areas in which it is in contact with such material, to lose its fiuxing properties. After the surfaces have been sprayed or similarly covered, the base member is fired to bond the electrically conductive material to the surfaces of the base member in the pattern areas 37 to form electrodes 39. During firing the hardened radiation sensitive material is burned off, leaving the electrically conductive material which covers it behind but free of the base member. On completion of the firing operation the loose electrically conductive material can be blown or brushed from the surfaces of the base member and the base member is then trimmed and cut into separate capacitor units as is well known in the art.

Referring now to FIGS. 15 and 16, an apparatus 40 is disclosed for use in any of the above described methods which facilitates properly aligning the masking means and positioning the base member therebetween so that the patterns on opposite sides of the base member are formed in a predetermined location relative to one another. As shown, the apparatus includes a rectangular frame 41 which is of Open construction, for a purpose to be hereinafter explained. A first masking means 42, having a plurality of radiation transmissive patterns 44 therein, which may be a photographic negative plate in those instances in which a light sensitive material is being used or a reflecting plate such as a mirror having heat transmissive openings in those cases in which a heat sensitive material is being used, is positioned on the frame 41 between guide or positioning strips 43 carried by the frame. Secured on top of the masking plate 42 in a predetermined relationship to the light or heat transmissive patterns 44 therein is a templet 45 having openings 46 for receiving and positioning the base members 47, on which the patterns are to be formed, in a predetermined manner relative to the radiation transmissive patterns of the masking means 42. A second masking means 48, having a plurality of radiation transmissive patterns 49 therein, is mov- ,ably positioned on top of templet 45 in overlying relation to masking plate 42 and is clamped in such position by clamping arms 50 pivotally connected to the frame.

In order to orient the masking means 48 relative to the masking means 42 so that the radiation transmissive patterns 44 and 49 are in a predetermined location relative to one another, means are provided for engaging the edges of the masking means 48 for shifting the same relative to the masking means 42. As shown, earns 51 are pivotally mounted on the frame 41 to engage the edges of the masking plate 48 and provide three adjustable index points for properly positioning the plate relative .to the masking plate 42. Each of the cams include an outwardly projecting arm 52, engaged by a spring means 53, carried on the frame, which resiliently bias the arm to rotate the cam in a clockwise direction. An adjustable stop means 54, carried by a bracket mounted on the frame, abuts the opposite side of each of the arms 52 for moving the arms against the pressure of their respective springs to shift the position of the cams in a counter-clockwise direction. It will be seen that by adjustment of the cams 51 the index points for the masking plate 48 can be moved as required to position the plate for properly aligning its radiation transmissive patterns with the radiation transmissive patterns of the masking plate 42.

The apparatus 40, when used in connection with any of the methods of making electrical units above described, is adapted to position a plurality of base members accurately between the masking means with the radiation sensitive surfaces of the base members adjacent the radiation transmissive patterns of the masking means for simultaneous exposure of the opposite surfaces of the base member to sources of radiation. Simultaneous exposure of the opposite surfaces of the base members, which substantially reduces the cost of manufacture and produces more accurate results, is facilitated by the open construction of the frame 41.

Thus, among others, the several objects and advantages of the invention as aforenoted are achieved. Obviously numerous changes in the method and structure may be resorted to without departing from the spirit of the invention as defined by the claims.

We claim:

1. The method of making an electrical unit comprising the steps of providing a base member having specific electrical properties, coating opposite surfaces of said base member with a radiation sensitive material adapted to harden on being exposed to light, said radiation sensitive material having suspended therein particles of electrically conductive material and a flux for bonding said particles to said base member, simultaneously controllably exposing at least a portion of each of said coated surfaces to radiation to harden the coating in said portions in predetermined patterns, said portions being predeterminately located relative to one another so that said patterns are similarly located, removing the unexposed coating, and then firing the unit to remove the hardened coating to leave the electrically conductive material suspended therein on said surfaces in said predetermined patterns and bond the same thereto.

2. The method of making an electrical capacitor comprising the steps of providing a base member of dielectric material, coating opposite surfaces of said base member with a light sensitive material adapted to harden on exposure to light, said light sensitive material having suspended therein particles of electrically conductive material and a flux for bonding said particles to said base member, simultaneously exposing at least a portion of each of said coated surfaces to light through a negative to harden the coating in said portions in predetermined patterns, said patterns on opposite surfaces being predeterminately located relative to one another, removing the unexposed coating and the electrically conductive material suspended therein, and then firing the capacitor to remove the hardened coating so that the electrically conductive material which has been suspended therein is left on said surfaces in said predetermined patterns and bonded thereto.

3. The method of making an electrical capacitor comprising the steps of providing a flat base member of ceramic dielectric material, coating opposite surfaces of said base member with a light sensitive material adapted to harden on exposure to light, said light sensitive material having particles of electrically conductive material and bonding material suspended therein, masking the opposite surfaces of said base member to define at least one coated area on each of said surfaces having a predetermined pattern, said patterns on opposite surfaces being predeterminately located relative to one another, exposing said coated area of each of said surfaces to light to harden the coating in said areas, removing the unexposed coating and the electrically conductive material suspended therein, and then firing the capacitor to remove the hardened coating and bond the electrically conductive material which had been suspended therein on said surfaces in said predetermined patterns.

4. The method of making an electrical capacitor comprising the steps of providing a fiat base member of ceramic dielectric material, coating opposite surfaces of said base member with a light sensitive material adapted to harden on exposure to light, said light sensitive material having particles of electrically conductive material and bonding material suspended therein, positioning a photographic negative having at least one light transmissive portion adjacent each coated surface of said base member, said light transmissive portions each defining a predetermined pattern with a pattern of one negative being predeterminately located relative to a pattern of the other negative when said negatives are positioned adjacent said base member, simultaneously exposing said coated surfaces to light'through said light transmissive portions of said negatives to harden the coating in said predetermined patterns, removing the unexposed coating and the electrically conductive material suspended therein, and then firing the capacitor to remove the hardened coating and bond the electrically conductive material which had been suspended therein on said surfaces in said predetermined patterns.

5. The method of making an electrical unit comprising the steps of providing a base member having specific electrical properties, coating opposite surfaces of said base member with radiation sensitive material of the character adapted to harden on being exposed to radiation, simultaneously exposing the opposite coated surfaces to radiation to harden the radiation sensitive material in such a manner as to define at least one predetermined pattern of unhardened coating on each of said surfaces, said patterns on opposite surfaces being predeterminately located relative to one another, removing the unhardened coating from said surfaces, covering the exposed surfaces with electrically conductive material, and firing the electrical unit to bond the electrically conductive material to said base member in the exposed pattern areas of said surfaces while removing said hardened coating and the electrically conductive material covering the hardened coating from said surfaces.

6. The method of making an electrical capacitor comprising the steps of providing a base member of dielectric material, coating opposite surfaces of said base member with a light sensitive material adapted to harden on being exposed to light, controllably exposing the opposite coated surfaces of said base member to light to harden the light sensitive material thereon in such a manner as to define at least one predetermined pattern on each of said surfaces of unhardened coating, said patterns on opposite surfaces being predeterminately located relative to one another, removing the unhardened coating from said surfaces, covering said opposite surfaces with electrically conductive material, and firing the capacitor to bond the electrically conductive material covering said patterns to said base member while removing the hardened coating and electrically conductive material covering the same from said surfaces.

References Cited by the Examiner UNITED STATES PATENTS 1,651,441 12/1927 Caps 65 2,193,333 3/1940 Heward 9565 2,443,119 6/1948 Rubin 204-18 2,506,604 5/1950 Lokker et a1 1568 2,699,424 1/1955 Nieter.

2,830,899 4/1958 Brown 9636 2,854,336 9/1958 Gutknecht 9636 2,977,228 3/1961 Gold et al. 9635 3,020,156 2/1962 Rowe 9635 3,031,302 4/1962 Bayer 9636 3,079,254 2/1963 Rowe 9636 3,080,232 3/1963 Gilbody et al. 9635 3,081,210 3/1963 Wolf et al. 1568 3,138,503 6/1964 Taraud 1568 XR 3,159,486 12/1964 Henderson 96-36 OTHER REFERENCES Guditz: Three Dimensional Printed Wiring, Electronics, vol. 30, June 1, 1957, pp. 160-163.

ALEXANDER WYMAN, Primary Examiner.

CARL F. KRAFFT, HAROLD ANSHER, EARL M.

BERGERT, Examiners. 

1. THE METHOD OF MAKING AN ELECTRICAL UNIT COMPRISING THE STEPS OF PROVIDING A BASE MEMBER HAVING SPECIFIC ELECTRICAL PROPERTIES, COATING OPPOSITE SURFACES OF SAID BASE MEMBER WITH A RADIATION SENSITIVE MATERIAL ADAPTED TO HARDEN ON BEING EXPOSED TO LIGHT, SAID RADIATION SENSITIVE MATERIAL HAVINGGSUSPENDED THEREIN PARTICLES OF ELECTRICALLY CONDUCTIVE MATERIAL AND A FLUX FOR BONDING SAID PARTICLES TO SAID BASE MEMBER, SIMULTANEOUSLY CONTROLLABLY EXPOSING AT LEAST A PORTION OF EACH OF SAID COATED SURFACES TO RADIATION TO HARDEN THE COATING IN SAID PORTIONS IN PREDETERMINED PATTERNS, SAID PORTIONS BEING PREDETERMINATELY LOCATED RELATIVE TO ONE ANOTHER SO THAT SAID PATTERNS ARE SIMILARLY LOCATED, REMOVING THE UNEXPOSED COATING, AND THEN FIRING THE UNIT TO REMOVE THE HARDENED COATING TO LEAVE THE ELECTRICALLY CONDUCTIVE MATERIAL SUSPENDED THEREIN ON SAID SURFACES IN SAID PREDETERMINED PATTERNS AND BOND THE SAME THERETO. 